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  \IEEEauthorblockN{Oliver Schmidt}
  \IEEEauthorblockA{TU Dresden\\
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Security, DRM, Trusted Execution Environments, SGX, 
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\section{Introduction}

\IEEEPARstart{S}{ince} early on in the so-called ``information economy'', publishers have tried to limit the distribution of their digital goods to the buyers only.  In his keynote ``The coming war on general computation'' at the 28th Chaos Communication Congress \cite{corydoctorowComingWarGeneral2011}, Cory Doctorow outlines the development of copy-protection mechanisms from first including uncopyable bad sectors to the floppy disks on which programs were distributed or tying the execution to dongles and license keys, to encrypted music and video files protected with dedicated \ac{DRM} schemes.  \\
But these attempts were mostly based on obscuring the protection mechanism used, thus being vulnerable to circumvention through reverse-engineering, patching out the protection mechanism or retaining the supposedly-secret media decryption keys even after the usage license expired.  The renowned IT security collumnist and expert Bruce Schneier commented on these attempts:  ``Digital files cannot be made uncopyable, any more than water can be made not wet.''   \cite{bruceschneierCryptoGramMay152001} The underlying vulnerability of all these copy protection schemes was that they were attempting to ``[\dots] figure out how to stop computers from running certain programs  and inspecting certain files and processes.''   \cite{corydoctorowComingWarGeneral2011} But as modern day computers are mostly general computation machines, they are inherently based on copying data and computing on them.  

Doctorow warns that all countermeasures trying to ensure copy protection of digital content are going to result in creating ``appliances'', which are still general purpose computers, but locked down with ``some combination of rootkits, spyware, and code-signing to prevent the user from knowing which processes are running, from installing her own software, and from terminating processes that she doesn't want.''   \cite{corydoctorowComingWarGeneral2011} \\
Additionally, international agreements based on the \ac{WIPO} Copyright Treaty \cite{worldinternationalcopyrightorganizationWIPOCopyrightTreaty1996}, its most prominent implementation being the US-American \ac{DMCA}, make the circumvention of ``technical protection measures'' such as \ac{DRM} illegal.  

So right now we are in a situation, where all major publishers of e.g. video\footnote{all Hollywood movie publishers require adhering to certain protec standards like \cite{movielabsinc.MovieLabsSpecificationEnhanced2018}} and audio\footnote{although legal music file purchases are mostly \ac{DRM}-free, the consumption model of streaming has brought back DRM to platforms like Spotify or Deezer} content, and video games\footnote{Valve's Steam platform integrates its own DRM into sold games; other publishers as well as gaming consoles have their own DRM systems as well}.  Thus right now, all software using this content has to be proprietary and whole platforms are being locked down more and more.  This development is most apparent in non-PC platforms like mobile devices, where unlocking the bootloader (if even possible) results in deletion of \ac{DRM} keys of the device. \cite{sonydeveloperworldUnlockBootloaderOpen}

But in recent years modern CPU architectures have introduced special hardware-backed \acp{TEE} to provide a secured environment for security-critical code to be executed in isolation from the main \textit{untrusted} \ac{OS}.  Can these TEEs and other special hardware trust-anchors provide the possibility to present \ac{DRM}-secured content in an otherwise open and open source system, not having to lock it down completely?

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\section{Background: Trust Anchors and Trusted Execution}
\label{sec:background}

This section first gives an overview about technologies used for having a trust anchor in a running system. Though these approaches are often used for locking down whole systems, they may also provide the basis for building more open systems with a \ac{TEE}.  \\
Afterwards we cover the technologies dedicated to provide a \ac{TEE} in modern processor architectures.

\subsection[SecureBoot]{UEFI Secure Boot}\label{sec:SecureBoot}

\textit{Secure Boot} is a functionality of the \ac{UEFI} boot firmware component \cite{unifiedefiforuminc.UEFISpecificationVersion2017} to allow only the launch of authenticated boot images.  To achieve that, boot images can be signed with X.509 certificates.  Only if the image verifies correctly against a key stored in non-volatile firmware memory or against an entry in an explicit allow list of signatures.  This first check on which bootloader or \ac{OS} image to launch can be the anchor of a trust chain, if each consecutive execution step also checks the authenticity of software to be launched.  The allow and deny lists can be updated from the running \ac{OS} and deploying own custom platform keys for verification can be possible through setup-mode of \ac{UEFI}.

Firmwares adhering to the \ac{UEFI} standard are the dominant system software for the PC platform (including x86 devices like servers, laptops and mobile devices, but also more and more devices with ARM chipsets).  Microsoft has been criticized for requiring devices \cite{MicrosoftHardwareCertification2014} shipped with the Windows \ac{OS} to have Secure Boot enabled by default, verifying boot images against Microsoft's key, mentioning that this could lead to lock-down of consumer hardware and the inability to install alternative operating systems on it.  \\
Earlier versions of the requirements mandated an option to disable Secure Boot on x86 devices while also mandating that it must always be enabled on ARM-based devices. \cite{moodyMicrosoftBlockingLinux}  Nevertheless complete lockout of other operating systems doesn't seem to be imminent at least on x86 devices as there is a shim bootloader signed by Microsoft \cite{matthewgarrettAnnouncingShimReview}, enabling the launch of other unsigned boot images.  The signature of this specific binary could though be put on the firmware's deny list through updates.

\subsection{TPM}\label{sec:TPM}

\acp{TPM} are dedicated hardware components offering a variety of security services to a system \cite{197213}:  \\
\acp{TPM} provide \textbf{secure storage} inside the module to store sensitive data like cryptographic keys in such a way that it can't be leaked to or stolen from processes running on the main CPU.  \\
These stored keys can be used to execute cryptographic operations on behalf of the system, without the keys getting transfered to the outside of the module. \\
The \textbf{attestation} functionality can attest the identity of software by calculating a hash sum of the supplied binary, signing it with its internal key to prove a certain system configuration or software version to third parties having a corresponding verification key.  When pre-deployed with a unique key, all previously mentioned functionality can be combined to provide a \textbf{unique machine identity} and form a hardware trust anchor for running systems. \\
A \textbf{continuous trust chain} can be built from the boot-up on if all software components, starting from the firmware on, let the \ac{TPM} attest the processes to be launched and compare these attestations with the ones stored previously in the module's secure storage.  A similar boot policy is \textit{authenticated booting}, where the \ac{TPM} calculates the checksum of each boot stage but doesn't enforce any signature checks but only stores the results inside its secure internal registers, from where the system status can be queried later on.\cite{hartigLateralThinkingTrustworthy2017}  In contrast to UEFI Secure Boot \ref{sec:SecureBoot},  it is also possible to securely attest and launch code using special \textit{late-launch} CPU instructions introduced into AMD and Intel chipsets without providing a continuous trust chain from the firmware on \cite{hartigLateralThinkingTrustworthy2017}.  \\
Additionally, \acp{TPM} provide important support functionality cryptographic algorithms like \textbf{secure counters}, a \textbf{secure clock} for peripherals and a \textbf{secure source of entropy}. \cite{197213}

lockdown techniques: TPM, secure boot

TEEs: SGX, TrustZone

(InkTag?)

Android DRM, EME DRM arch, TBDRM (-> fTPM)


\section{lockdown/ openness}

What parts need to be locked down?

WiDevine/ Netflix ban from rooted devices

- remaining other problems: cultural heritage, inflexibility, tied to a certain technological platform

Doctorows Law: "Anytime someone puts a lock on something you own, against your wishes, and doesn't give you the key, they're not doing it for your benefit."

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